Random glimpses of an irrelvant future that will no longer come to pass are entirely uninteresting,
Which is why the writers shouldn't have invalidated predestination because they just made their premise uninteresting. But if they wanted to do that, they should go with it, not pretend they never did it. What incoherent writing. Choose a premise and stick with it!
If the show centered on a non-Western society, I'd be quicker to believe that the characters see the flashes as omens. But in a Western society, particularly America, people are very resistant to the idea of not being in control of their lives. As far as we know, our lives are governed by predestination now, yet how many Americans choose to believe that? I've never met a single one. And I'm supposed to believe in a bunch of American characters who suddenly change the cultural habits of a lifetime? It's far more likely that, once given an "out," they will quickly return to their accustomed mindset that predestination is bunk.
My observation of human nature is that people make up their minds how to live and are incredibly resistant to change. You can wave proof of something in front of their faces and they'll just ignore it or make up a reason why it's wrong or doesn't apply to them. (As a fellow habitue of TNZ, I'm sure you see this on a daily basis. Or turn on Fox News for further proof.)

Some weird hallucination might disorient them for a while, but they'd quickly forget it and return to their old habits. I'd bet a lot of people would had done that even if Gough hadn't given them an excuse. This premise assumes people are far more rational than they really are, and far less driven by habit and delusion.
The possibility that they are merely a glimpse of "the" future was refuted by Mark burning the friendship bracelet, as well as Gough's suicide.
The friendship bracelet didn't prove anything since the daughter could have made another one. Gough's suicide was the first definitive proof that predestination wasn't governing their lives.
Lastly, Mark Benford is a genuinely flawed hero. He's an alcoholic who is choosing between his marriage and sobriety and possibly his very life, or finding out the solution to the mystery but he's still thinking about it!
Since anything can happen, Mark could figure out the flash forward mystery and stay sober as a deacon. There's no reason to believe his flash forward has a certain probability of coming true. It might have less probability of coming true than him being hit by lighting on that particular day. And presumably he isn't letting the fear of random, but possible, fatal accident rule his life so why should he give a second thought to a flash forward that may not have anything to do with anything?
Yeah but Gough's suicide isn't the first one, merely the first of the characters to do it.
He was the first person to commit suicide in order to disprove predestination (or perhaps the first who managed to communicate his intent). Without using the suicide to send that message, the person committing suicide wouldn't receive the attention they craved. The attention is the point of the suicide.
And someone would have craved that attention long before Gough did. With 6.5 billion people on the planet, I wouldn't be surprised if potential Goughs numbered in the tens of thousands. It's really not so rare for suicidal people to want to make some statement with their death that will grant them immortality.
Still there's no reason for it to affect Bryce
He's behaving as though he can ignore his cancer, that it won't kill him or incapacitate him at any moment. Then again, he was suicidal, so perhaps he simply doesn't care what happens to him.
Mark, Demitri and Olivia are bigger problems - why are they acting like they can't easily sidestep their terrible futures? Demitri can't be "murdered" unless he's around people - just go somewhere that he's a thousand miles away from the nearest human or lock himself into a vault for the whole day.
Mark could simply decide not to go into work that day and his flash forward can't happen. Is someone going to drag him into work kicking and screaming? Why doesn't he just go on vacation to Cancun, or Antarctica, so that he's thousands of miles away on that day?
Olivia decides she isn't going to cheat on her husband, period. Problem solved. And the babysitter goes on vacation in the Sahara, where it is physically impossible to be drowned due to total lack of water. These people need to use their imaginations more, or the writers do.
Most of these characters are making their future happen despite what they saw in their visions.
Mark, Olivia, Demitri and the babysitter whose name I still haven't learned have a motive to avoid their futures. They just aren't using their heads.